Stainless steel having stress corrosion crack resisting property



Aplll 15, 1969 MASAO KAWAHATA ET AL 3,438,769

STAINLESS STEEL HAVING STRESS CORROSION CRACK RESISTING PROPERTY Filed Dec. a, 1966 Sheet of 2 Stress corrosion crack/ 23 time (hr) 0 5 I0 is 20 2'5 C'lzromium. content STAINLESS STEEL HAVING STRESS CORROSION CRACK RESISTING PROPERTY 1 5, 1969 MASAO HAT L- ZT AL 3,438,769

Filed Dec. 8, 1966 Sheet 2 Of 2 Stress corrosion crack/11y time (/Zr) I l I l I I o 0.5 i 115 2 2.5 a 3.5

Silicon content United States Patent 3,438,769 STAINLESS STEEL HAVING STRESS CORROSION CRACK RESISTING PROPERTY Masao Kawahata, Yokohama, and Yukishige Fukase and Koichiro Osozawa, Kawasaki, Japan, assignors to Nippon Yakin Kogyo Company Limited, Tokyo, Japan, a corporation of Japan Filed Dec. 8, 1966, Ser. No. 600,240 Claims priority, application Japan, Dec. 15, 1965, 40/ 76,671 Int. Cl. C22c 39/20, 39/26, 39/48 US. Cl. 75-125 2 Claims The present invention relates to stainless steel having stress corrosion crack resisting property, which contains as it main ingredients 0.0l0.l% by weight of carbon, Gill-3% by weight of manganese, 1240% by weight of nickel, 0.00l0.5% by weight of molybdenum, 848% by weight of chromium, 2-6% by weight of silicon, 16% by Weight of copper, and remainder of iron and also stainless steel which further contains ODS-2% by weight of one or more than two of elements selected from a group consisting of niobium, tantalum and titanium.

The object of the invention is to provide austenitic or austeniiic-ferric-stainless steel having an excellent stress corrosion crack resisting property under various environments containing halide, particularly chloride.

In general, austenitic stainless steel is used under the environment containing chloride as various constitutional material or as apparatus for use in various kinds of chemical industry, petroleum industry and nuclear energy industry, In such a case, the austenitic stainless steel has a tendency of becoming subjected to a local corrosion such as pitting corrosion, stress corrosion cracking and crevice corrosion.

In this case, even if the apparatus is not yet subjected to corrosion as a whole, but the local corrosion renders it impossible to use the apparatus. The stress corrosion cracking is produced under the environment containing chloride if the stainless steel is subjected to the internal or external tensile stress. A number of accidents occurred due to the above corrosion have been known, but the fundamental solution thereof could not be found out.

In order to solve the above problem concerning the stress corrosion cracking, it has been proposed to remove the tensile stress subjected to the stainless steel internally or externally, to control the environment and to improve the stainless steel to be used. The inventors have investigated for many years on the influence of various elements upon the stress corrosion cracking property of the stainless steel for the purpose of improving the stress corrosion crack resisting property of the stainless steel. The inventors found out as the result of the above investigation that chromium, molybdenum and nitrogen are injurious to the above resisting properties of the stainless steel, while carbon, silicon and nickel are effective to improve said resisting property and copper is not injurious to said resisting property.

It has been well known that the stress corrosion crack resisting property of the austenitic stainless steel can be improved by increasing the nickel content thereof. In this case, it is necessary to add more than 40% by weight of nickel in order to obtain a satisfactory stress corrosion crack resisting property. However, much amount of nickel in the stainless steel results in a remarkable high expense thereof, in difliculty in case of welding and hot working etc. The sensitivity of the stainless steel to the stress corrosion cracking can be reduced to a more or less extent by decreasing the ntirogen content in the stainless steel with the aid of vacuum melting process. The process, however,

"ice

has disadvantages that the production cost becomes remarkably high.

One of the characteristic features of the stainless steel according to the present invention is that the stress corrosion crack resisting propert thereof is improved by decreasing the chromium content to a value which is substantially equal to or lower than the lower limit of the chromium content of the conventional austenitic stainless steel.

The invention will be explained with reference to the accompanying drawings, in which;

FIG. 1 is a diagram illustrating the influence of the chromium content in the stainless steel containing 20% by weight of nickel upon the stress corrosion cracking time thereof; and

FIG. 2 is a diagram illustrating the mutual relations between the stress corrosion cracking time of the stainless steel and the silicon content thereof.

FIG. 1 shows stress corrosion cracking time measured when various amounts of chromium are added to the stainless steel containing 20% by weight of the nickel as is main ingredient. The stress corrosion cracking test was carried out in boiling aqueous solution of magnesium chloride at a temperature of C. The: test results obtained by subjecting the tensile stress of 25 kg./mm. to the stainless steel are shown by symbol and the test results obtained by subjecting the tensile stress of 35 kg./ mm? to the stainless steel are shown by symbol The stainless steel having the cracking time or the time of breakage longer than 300 hours is considered to be one having practically sufficient stress corrosion crack resisting property. The stress corrosion crack resisting property is remarkably increased with decrease of the amount of the chromium added in the stainless steel. Thus, the stainless steel has to contain not more than certain upper limit of the amount of chromium in order to have an excellent stress corrosion crack resisting property. The lesser the chromium content is the more excellent stress corrosion crack resisting property is obtained. But, the general corrosion property of the stainless steel containing less than 8% by weight of chromium is deteriorated thus losing the inherent property of the stainless steel.

The second characteristic fea ure of the s ainless steel according to the invention is that it is added with much amount of silicon for the purpose of improving the stress corrosion crack resisting property. Thus, the sensitivity of the stainless steel to the stress corrosion cracking can be reduced even if the stainless steel contains a slight amount of injurious elements.

FIG. 2 shows mutual relations between the amount of silicon added to the stainless steel and the stress corrosion cracking time. It is clearly shown that the addition of more than 2% by weight of silicon is particularly effective. The test was carried out under the same conditions as in the case described with reference to FIG. 1. The tensile stress of 25 kg./mm. shown by symbol and the tensile stress of 35 kgmm shown by symbol were subjected to the stainless steel and the time up to the breakage was measured. Cr-Ni stainless steel was used as the test material.

The stainless steel containing lesser amount of chromium and not containing elements for improving the general corrosion property such as molybdenum is liable to be subjected to the general corrosion or pitting corrosion if the stainless steel is used under environment containing chloride. Thus, this stainless steel loses its inherent property as stainless steel even if it has the stress corrosion crack resisting property.

The third characteristic feature of the stainless steel according to the invention is that the stainless steel having stress corrosion crack resisting property obtained by the addition of a limited amount of chromium and much amount of silicon is further added with l6% by weight of copper for the purpose of obtaining an excellent general corrosion resisting property. This addition of copper makes it possible to provide a stainless steel which has both very good stress corrosion crack resisting property and the general corrosion property under various kinds of environments containing chloride.

The following Table 1 shows the test results obtained by subjecting the sample stainless steel according to the invention and conventional stainless steel to 25 kg./mm.

of stress and 35 kg./mm. of stress in the testing liquid 1 TABLE 2 CONVENTIONAL STAINLESS STEEL Rate of corrosion (g./rn.z. hr)

5% sulphuric acid 0.5% hydrochloric (6 hr.) acid (0 hr.)

Sample notation:

STAINLESS STEEL ACCORDING TO THE INVENTION TABLE 1.Test Results of Stress Corrosion Cracking of Conventional Stainless Steel and Stainless Steel According to the Invention.

CONVENTIONAL STAINLESS STEEL Compositions (weight percent) Stress corrosion cracking 1 More than 300.

As can be seen from the above Table l, the stainless steel according to the invention does not produce any cracking at all during the testing time up to 300 hours and shows the excellent property thereof.

It is found that the stainless steel according to the invention is excellent in the general corrosion resisting property and that it is not influenced by corrosion in the above mentioned stress corrosion testing liquid. The stainless steel according to the invention is superior in the corrosion resisting property to the conventional 18-8 stainless steel available in market when it is subjected to corrosion test using 5% sulphuric acid corrosion test, defined by the Japanese Industrial Standard or to immersion test using diluted hydrochloric acid, which are more severe than the above mentioned corrosion testing liquid.

The stainless steel according to the invention shows the general corrosion resisting property which corresponds to that of the stainless steel containing molybdenum.

The following Table 2 shows examples of the rate of corrosion in g./m. -hr. obtained by the immersion tests in 5% sulphuric acid for 6 hours and in 0.5% hydrochloric acid for 6 hours.

used as a constructional material which could not be subjected to the solid solution heat treatment after having been heated by means of the welding treatment or other heat treatment, the stainless steel according to the present invention has to contain less than 0.03% by weight of carbon or contain a small amount of one or more than two of elements selected from a group consisting of niobium, tantalum and titanium so as to stabilize the carbon in the stainless steel. This renders it possible to fully utilize the stainless steel according to the present invention without losing the inherent property thereof.

The reason why the compositions of the stainless steel according to the present invention are limited to the above mentioned range is as follows:

(1) 0.0l0.l% by weight of carbon The stainless steel containing less than 0.01% by weight of carbon is difiicult to be manufactured by a melting process in air and the stress corrosion crack resisting property thereof becomes deteriorated. Carbon is necessary for giving the stress corrosion crack resisting property to the stainless steel. But, the stainless steel added with more than 0.1% by weight of carbon becomes sensitive to the grain boundary corrosion whereby the desired efiect could not be attained.

Manganese is indispensable for manufacturing the stainless steel as a deoxidizing element and introduced incidentally into the stainless steel as impurities. In this case, the content of up to 3% by weight of manganese is permitted. The lower limit of 0.01% by weight of manganese is the limit where the stainless steel can be manufactured by means of the melting process in air.

(3) 0.0010.5% by weight of molybdenum Molybdenum is an element normally included in the stainless steel and is injurious to the stress corrosion crack resisting property thereof. 0.001% by weight of molybdenum is the lower limit permitted to be included in the stainless steel from the raw material. More than 0.5% by weight of molybdenum deteriorates the stress corrosion crack resisting property of the stainless steel according to the invention.

(4) 12-40% by weight of nickel Nickel is an element necessary for giving the stress corrosion crack resisting property and general corrosion resisting property to the stainless steel according to the invention.

At the same time nickel serves to stabilize the structure of the stainless steel, and also to make the manufacture and working of the stainless steel easy. Thus, the lower limit of the nickel content is determined as 12% by weight. The higher the upper limit is the more improvement is obtained with respect to the stress corrosion crack resisting property of the stainless steel. But, as the upper limit becomes higher it makes the stainless steel expensive so that the upper limit is determined as 40% by weight from the point of economy.

Silicon is an element which remarkably improves the stress corrosion crack resisting property of the stainless steel. Less than 2% by weight of silicon makes impossible to sufiiciently increase the stress corrosion crack resisting property if injurious elements are included in the stainless steel. However, more than 6% by weight of silicon remarkably deteriorates forgeability and weldability of the stainless steel thus making the manufacture thereof difiicult.

(6) 8-18% by weight of chromium Chromium is an element injurious to the stress corrosion crack resisting property of the stainless steel. Less than 8% by weight of chromium causes the steel to produce the general corrosion and pitting corrosion in the chloride solution. More than 18% by weight of chromium 0.013% by weight of manganese 2-6% by weight of silicon increases sensibility of the stainless steel to the stress corrosion cracking.

Copper is added to the stainless steel according to the invention for the purpose of maintaining the general corrosion resisting property thereof. Less than 1% by weight of copper is less effective. More than 6% by Weight of copper segregates a phase having copper as its main ingredient thus deteriorating the corrosion resisting property and the stability of the system of the stainless steel and making the manufacture of the stainless steel difiicult.

Niobium, tantalum and titanium are not required to be added if annealing of the stainless steel has been completed and the stainless steel contains less than 0.03% by weight of carbon so that there is no danger of becoming subjected to the grain boundary corrosion.

However, in general 0.05 2% by weight of these elements may preferably be added. Less than 0.05% by weight of one or more than two of these elements selected from a group consisting of niobium, tantalum and titanium is not effective to the grain boundary corrosion resisting property. More than 2% by weight of these elements produces a compound of these elements with another elements, thereby making the structure of the stainless steel unstable and making the manufacture thereof dilficult.

What We claim is:

1. Stainless steel having stress corrosion crack resisting property, which contains as its main ingredients 0.01- 0.1% by weight of carbon, 0.01-3% by weight of manganese, 0.0010.5% by weight of molybdenum, 12-40% by weight of nickel, and 8-18% by weight of chromium and further contains 2-6% by weight of silicon and 16% by weight of copper, remainder being chiefly iron and slight amount of impurities.

2. Stainless steel as claimed in claim 1, which further contains 0.052% by weight of the elements selected from the group consisting of niobium, tantalum and titanium.

1-6% by weight of copper Niobium, tantalum and titanium References Cited UNITED STATES PATENTS 2,251,163 7/1941 Payson -128 2,984,563 5/1961 Tanczyn 75-1289 3,044,871 7/1962 Mott 75125 3,366,473 1/1968 Nehrenberg.

HYLAND BIZOT, Primary Examiner.

US. Cl. X.R. 

1. STAINLESS STEEL HAVING STRESS CORROSION CRACK RESISTING PROPERTY, WHICH CONTAINS AS ITS MAIN INGREDIENTS 0.010.1% BY WEIGHT OF CARBON, 0.01-3% BY WEIGHT OF MANGANESE, 0.001-0.5% BY WEIGHT OF MOLYBDENUM, 12-40% BY WEIGHT OF NICKEL, AND 8-18% BY WEIGHT OF CHORMIUM AND FURTHER CONTAINS 2-6% BY WEIGHT OF SILICON AND 1-6% BY WEIGHT OF COPPER, REMAINDER BEING CHIEFLY IRON AND SLIGHT AMOUNT OF IMPURITIES. 